The invention concerns a process and an apparatus for the automatic measuring of a workpiece carried on a coordinate table movable in at least two mutually orthogonal directions by scanning the workpiece by at least one stylus of a scanning head, during a relative movement between the workpiece and the scanning head in one coordinate direction.
Processes and devices of this type are important in the case of three coordinate measuring machines whereby the profile of a workpiece is to be scanned and wherein the scanner and the workpiece are to be maintained in permanent contact by means of suitable control measures.
A process and an apparatus for this purpose are known from DE-OS No. 26 54 839, whereby an absolute measured value is determined for the deflection of the scanning stylus produced during the scanning of the workpiece and wherein by the addition with the correct sign of the measured value of the stylus deflection and of a measured value indicating the position of the measuring head, a value characterizes each contour location of the workpiece. Wherein further measured values are determined such that the workpiece or the measuring head is always moved in a direction that is laterally offset with respect to the deflection of the stylus and by that during the peripheral scanning of the workpiece the scanning stylus is constantly deflected from its base position.
U.S. Pat. No. 3,394,248 to Ogden further discloses a system for the measurement of the magnitude of a relative motion between an object and a reference point, wherein optical gratings are used in order to convert the motion, by means of photoelectrical elements, into electrical pulses, which depending on their direction of motion are fed into a forward/backward counter by means of an addition or subtraction stage.
A type of measuring machine is also known, wherein control signals are derived from the signals of an induction scanner which represent the deflection of the scanning head in mutually orthogonal directions. The signals are utilized in a control circuit, which maintains a predetermined contact pressure during relative motion of the workpiece and the scanning head in one coordinate direction.
However, the above-mentioned measuring machines have the common disadvantage that in the vicinity of the point of switching from one measuring coordinate direction into the other measurements become inaccurate as the result of the drag error generated by the friction between the workpiece and the scanner.
Another disadvantage common to the known measuring machines is that the scanning process is interrupted during the switching, because first the relative motion between the scanning head and the workpiece in one coordinate direction must be stopped before the relative motion in the other direction may be initiated. The error caused by braking and acceleration prevents the acquisition of measuring values within a certain range.